I'm afraid I can't help all that much, but since nobody else does I'll try to do what I can.
I'm very familiar with the SNES audio compression format, BRR, as I've written a WAV->BRR and BRR->WAV converted. Seach the Utilities sections for "BRRTools".
The sound processor used in the SNES was made by Sony, and the sound units in the PS1 and the PS2 are direct sequels of it. The PS1 uses a similar format called VAG/VAB. I've always been willing to add it to BRRTools, but was too lazy to do so so far. Pehaps someday.
Basically the main difference is that it uses blocks of 2-byte header and 28 samples (stored as nibbles in 14 bytes) totalizing in 16 bytes blocks (the SNES uses 1-byte header + 16 samples : total 9 bytes).
The idea however is the same : There is a shift count, and a prediction (aka filter) lookup number + a loop and end flag in the header of each sample.
I don't know how much the PS2 format differes from the PS1 (if it does at all) but that's already a start.
PS : Anyways, you should search for a tool called "PSound". This converts PS1 audio, but could also work with PS2 if the format is the same ?